Most modern residential smoke detectors employ piezoelectric alarm horns to effect audible supervision signals and products-of-combustion alarm warnings. A piezoelectric horn generally comprises an acoustic chamber formed by an annular wall integral with a smoke detector cover. A piezoelectric disc is seated upon the wall to close the chamber internally; and the smoke detector cover is formed with sound openings from which audible signals emanate.
Electrical connections must be established with the piezoelectric disc to complete an oscillator circuit which drives the disc into a sound-generating vibratory state. In general, these electrical connections to the piezoelectric disc are not hardwired, but are manually separable. For example, the connections may be effected by terminals supported on a circuit board fixed to a smoke detector base, and the piezoelectric disc is fixed to an acoustic chamber integral with the smoke detector cover. When the smoke is assembled, the necessary circuit connections to the piezoelectric disc are established.
A two-piece assembly employing a metal bead pin supporting a coil spring is commonplace in prior-art terminals applied to piezoelectric discs used in smoke detector horns. The bead pin is inserted into a circuit board with a bench press. The circuit board has a narrow tolerance hole diameter to insure that the bead pin fits tightly when forced into the board. In actual practice, several bead pins are first loaded into a fixture that inserts them into a board. This is a laborious process. At the outset, this was done manually by an operator. At a later time, a method was devised by which several pins were vibrated over the surface of the circuit board, and eventually most or all of the pins would fall by themselves into the proper holes.
Once all of the pins were inserted into a production circuit board, the board would be stuffed with the remaining necessary electronic components and thereafter wave soldered. After passing an electrical functional test, the circuit board would be transferred to final assembly where an extremely small coil spring contact would be placed manually on each bead pin. An assembled coil spring on a bead pin would constitute a spring contact for a piezoelectric disc.
Missing coil springs and springs misaligned due to improper insertion on the bead pin were commonplace production problems.
When the smoke detector is assembled, the springs are in contact with a side of the piezoelectric disc. Since the plastic cases of smoke detectors are relatively flexible, being made of molded elements that are less than 0.08 inches thick, user manipulation, by squeezing or pressing the smoke detector housing, would cause the springs to rub on the piezoelectric disc. This action would cause a characteristic scratchy sound which was puzzling to the user and which led to the impression that the smoke detector was of cheap quality.